This invention relates to computer architecture and more particularly to the arrangement of circuit boards or chips in a computer or other highly-complex electronic circuit so as to maximize component density and minimize conduction path length to increase computing capacity and speed of operation.
A long-standing objective of computer design has been to increase both computing capacity and the speed of operation of computers of all sizes. In an article entitled "Supercomputer," Scientific American, January, 1982, pages 120-135, Ronald D. Levine details various attempts at attaining these goals in the very largest computers. The latest supercomputers are the CRAY-1 built by Cray Research, Inc. and the CYBER 205, built by Control Data Corporation. A variety of programming and structural innovations have been employed in these machines to maximize capacity and speed of operation.
Current performance levels of each machines--computing speeds in excess of 100 million operations per second--are due in significant part to the rapid advance of microelectronics, which through miniaturization has made available very dense memory and logic circircuitry having nanosecond switching speeds. At such high switching speeds, the speed of circuit operation is limited primarily by the speed at which signals can be propagated from one part of the machine to another. No signal can travel faster than the speed of light, which is close to one foot per nanosecond. In practice, the propagation speed of pulses through the wiring of supercomputers ranges from 0.4 to 0.9 feet per nanosecond. To reduce the cycle time of a computer to one nanosecond requires that the distance separating synchronous parts of the machine be appreciably less than one foot. The cycle times of existing supercomputers are roughly proportional to their linear dimensions, evidence that signal propagation speed is the primary limiting factor. In the CRAY-1 computer, average wire length is about four feet and cycle time is 12.5 nanoseconds. In the CRAY-2 computer, recently announced as being in the developmental stage, average wire length is 16 inches. It is expected to have a cycle time of four nanoseconds, still much slower than the switching speeds of the components. It would be desirable to further reduce conductor length to the point where signal propagation times are not a limiting factor in speed of operation.
Besides miniaturization, a variety of stacking techniques have been proposed to concentrate large numbers of circuits in small volumes. U.S. Pat. No. 3,087,096 to Jorgensen and U.S. Pat. No. 3,243,661 to Ullery, Jr. propose stacking parallel wafers in columns and electrically interconnecting them along the sides of the column. U.S. Pat. No. 2,872,664 to Minot and U.S. Pat. No. 3,704,455 to Scarbrough propose similar schemes for stacking and interconnecting computer memory circuit boards. Although the increasing density of integrated circuits has enabled substantial size reductions, the miniaturization of supercomputers is inhibited by the vast amount of memory and logic circuitry they require. In the CRAY-1 computer, more than 1,600 circuit boards in the machine's central processing unit are stacked in columns which, in turn, are arranged side by side in a cylindrical array. The circuit boards are interconnected inside the array by about 300,000 wires. The wires in each machine must be adjusted in length so that the signals between any two points deviate from a desired travel time by less than one nanosecond. It would be preferable to minimize such pains-taking tuning requirements.
In addition, concentrating a large amount of circuitry in a small volume to minimize the length of the wires creates a serious problem: the removal of waste heat generated by electrical energy conversions. Heat removal is critical because semiconductor failure rates increase rapidly with temperature. The aforementioned Levine article describes various techniques employed in supercomputers to provide adequate cooling and mathematical techniques used to detect and correct bit errors in the machines. The CRAY-2 computer employs liquid immersion technology to remove heat from its densely packed circuit modules. The application of cryogenics to computers has also been proposed in, for example, an article entitled "The Super-conducting Computer" by Juri Matisoo, Scientific American, May 1980, pages 50-65. Accordingly, it is important that arrangements of the chips or boards avoid localized heat concentrations and permit free flow of cooling fluid.
It is also desirable to simplify the electrical interconnection of boards or chips in complex circuits, particularly to avoid the maze of wiring used, for example, in the CRAY-1 computer. Matisoo proposes stacking the boards or cards to which the circuit chips are bonded in a parallel, spaced array and interconnecting the edges of the cards via perpendicular wiring modules. Electrical contact between the cards and the wiring modules is provided by solder joints in which a drop of mercury bridges a gap between two micropins, one in the card and the other in the wiring module. Whn the computer is immersed in liquid helium, the mercury solidifies. It would be preferable to minimize the need for either extrinsic wiring or separate wiring modules.